Clause 3 - Abbreviations and definitions

There are a number of new abbreviations and terms in ANSI/ISA-84.00.01-2004-1. There is also clarification or expansion of some definitions associated with terminology used in ANSI/ISA-84.01-1996. 

Perhaps, the most significant new term is Safety Instrumented Function (SIF). In ANSI/ISA-84.01-1996, there was very little differentiation between an SIS and SIF. ANSI/ISA-84.00.01-2004-1 spells out the difference between the SIF (e.g., hardware software used to mitigate a specific process risk) and the SIS (i.e., the hardware and software used to implement multiple SIFs). For example, consider a process furnace. If the main fuel gas pressure to the burner is too high, the flame may blow out, allowing uncombusted gases to be introduced into the furnace with the potential for a fire within the furnace. To prevent this hazard, an SIF is implemented in which block valves are used to isolate the main fuel gas from the process furnace when high fuel gas pressure is detected. These same main fuel gas valves may also be closed in response to low pilot gas pressure (i.e., a different process hazard). The SIS is the implementation of these two SIFs in the same logic solver. 

The term prior use is used in ANSI/ISA-84.00.01-2004-1. This term is applied when the owner/operator is justifying the implementation of a device in an SIF application based on prior use of the device in a similar operating profile. The standard provides specific requirements for assessing prior use for devices based on the device type, safety integrity level, and, in the case of PE logic solvers, the safe failure 

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